Nearly all men with metastatic prostate cancer will respond favorably to androgen deprivation therapy (ADT), the mainstay treatment since the 1940s. Yet, metastatic prostate cancer remains incurable because all patients eventually experience disease relapse in an ADT resistant form call castration resistant prostate cancer (CRPC). Mechanisms of ADT resistance converge on three general forms, reactivation of androgen receptor (AR) signaling, compensatory signaling via the glucocorticoid receptor, or phenotypic transformation to an AR signaling indifferent state via lineage plasticity. The first two forms retain their dependence on AR or compensatory signaling (ARhi), thus therapeutic development is focused in improving signaling blockade. CRPC mediated by lineage plasticity (ARlo), however, is not well understood. This knowledge gap has hindered development of therapies to counter it. We have recently demonstrated that EZH2 inhibitors can reverse ARlo CRPC and restore ADT sensitivity. EZH2 may also function as an AR coactivator, driving ARhi CRPC. These observations suggest the hypothesis that EZH2 suppression may reverse epigenetic reprogramming contributing to both ARhi and ARlo CRPC. Thus EZH2 inhibitors could be used to prolong beneficial ADT clinical responses. The goals of this application are to test the hypothesis, elucidate underlying mechanisms, and identify additional epigenetic vulnerabilities unique to ARhi and ARlo CRPC. We propose two specific aims to advance these goals: 1) Characterize effects of EZH2 suppression on ARhi and ARlo CRPC; 2) Identify epigenetic vulnerabilities unique to ARlo or ARhi CRPC. The work is possible with resources available through the MSKCC-UW/Fred Hutch Prostate Cancer Drug Resistance and Sensitivity Center.